Fiber reinforced composite materials typified by fiber reinforced plastics (FRP), in particular, carbon fiber reinforced plastics (CFRP), have large tensile strength and tensile elastic force and small density, as compared with glass fiber reinforced plastics (GFRP), aramid fiber reinforced plastics (AFRP), or stainless steel (SUS), and have therefore tended to be frequently used as outer panels or the like of aircrafts or vehicles in recent years. In this context, CFRP refers to plastics prepared by hot molding or hot press molding one or two or more layers of prepregs containing carbon fiber impregnated with a matrix resin. A member formed from this CFRP is fixed to a structure using a fastening factor such as a bolt or a rivet. Therefore, machining work, in particular, machining work to make many holes for passing a fastening factor in CFRP, is necessary for fixing CFRP to a structure such as an aircraft part.
Some techniques have already been proposed in order to obtain high quality holes by the machining process of CFRP. Examples of such a technique include a method which involves gradually changing the shape of a tool, for example, the curvature of a machining face or the point angle of a drill (see, for example, Patent Literature 1).
Materials for the bodywork structures (structural materials) of aircrafts are composed mainly of metal materials, most of which are occupied by aluminum alloy. Heat resistant alloy such as titanium alloy or stainless steel is used in a site capable of becoming a higher temperature, for example, a jet exhaust site or the neighborhood of an afterburner, in a bodywork structure. If the speed up of aircrafts will progress in the future, the strength of conventional aluminum alloy will be reduced due to aerodynamic heating. Therefore, it is expected that harder titanium alloy or stainless steel will be used as main structural materials for bodywork structures. These structural materials constituting the bodyworks of aircrafts need to undergo drilling work with a drill for bolting metal materials or a metal material with another structural material such as CFRP.
Some techniques have already been proposed for such drilling work of metals. For example, a titanium alloy material is a difficult-to-machining material and therefore shortens a drill lifespan very much. In response to such a problem, for example, a processing method which involves spraying a machining oil, and a method which involves changing the shape of a drill to thereby reduce load to the drill and circumvent reduction in the lifespan of the drill are listed (see, for example, Patent Literature 2).